In general, a spark plug includes a center electrode and a ground electrode that are disposed at a distal end side of the spark plug. The center electrode projects from a distal end of an insulator and is held in an axial hole of the insulator. On the other hand, the ground electrode is sealed to a distal end portion of a metal shell.
One of properties required for a spark plug is the breakage resistance of the ground electrode. Conventionally, a variety of techniques have been proposed in order to enhance the breakage resistance of the ground electrode (refer to the following Patent Literatures).
JP-A-2013-222676 discloses a technique that enhances the breakage resistance of the ground electrode by disposing a large width portion at a portion of the ground electrode. JP-A-2013-012462 discloses a technique that enhances the breakage resistance of the ground electrode by adjusting the thickness of the ground electrode in the radial direction. JP-A-2012-160351 discloses a technique that enhances the breakage resistance of the ground electrode by disposing a depressed portion at a back surface or a side surface of a bending portion of the ground electrode, and increasing the hardness of a bottom of the depressed portion. JP-A-2010-80059 discloses a technique that enhances the breakage resistance of the ground electrode by disposing a needle-shaped electrode tip in the ground electrode.